Inheritance (Part 2) Notes Chapter 6 1
Object Dog extends Object Dog PureBreed extends Dog PureBreed Mix BloodHound Komondor... Komondor extends PureBreed 2
Implementing Inheritance suppose you want to implement an inheritance hierarchy that represents breeds of dogs for the purpose of helping people decide what kind of dog would be appropriate for them many possible fields: appearance, size, energy, grooming requirements, amount of exercise needed, protectiveness, compatibility with children, etc. we will assume two fields measured on a 10 point scale size from 1 (small) to 10 (giant) energy from 1 (lazy) to 10 (high energy) 3
Dog public class Dog extends Object { private int size; private int energy; // creates an "average" dog Dog() { this(5, 5); } Dog(int size, int energy) { this.setsize(size); this.setenergy(energy); } 4
public int getsize() { return this.size; } public int getenergy() { return this.energy; } public final void setsize(int size) { this.size = size; } } public final void setenergy(int energy) { this.energy = energy; } why final? stay tuned 5
What is a Subclass? a subclass looks like a new class that has the same API as its superclass with perhaps some additional methods and fields the new class has direct access to the public and protected* fields and methods without having to redeclare or re-implement them the new class can introduce new fields and methods the new class can re-define (override) its superclass methods 6 * the notes does not discuss protected access
Mix UML Diagram a mixed breed dog is a dog whose ancestry is unknown or includes more than one pure breed Dog Mix 1 breeds ArrayList<String> 7
Dog - size : int - energy : int + setsize() + setenergy() + equals(object) : boolean + hashcode() : int + tostring() : String... Mix - breeds : ArrayList<String> + getbreeds() : List<String> + equals(object) : boolean + hashcode() : int + tostring() : String... subclass can add new fields subclass can add new methods subclass can change the implementation of inherited methods 8
What is a Subclass? a subclass looks like a new class that has the same API as its superclass with perhaps some additional methods and fields inheritance does more than copy the API of the superclass the derived class contains a subobject of the parent class the superclass subobject needs to be constructed (just like a regular object) the mechanism to perform the construction of the superclass subobject is to call the superclass constructor 9
What is a Subclass? another model of inheritance is to imagine that the subclass contains all of the fields of the parent class (including the private fields), but cannot directly use the private fields 10
Mix Memory Diagram belongs to superclass private in superclass not accessible by name to Mix 500 Mix object size 1 energy 10 breeds 1000 11
Constructors of Subclasses the purpose of a constructor is to set the values of the fields of this object how can a constructor set the value of a field that belongs to the superclass? by calling the superclass constructor and passing this as an implicit argument 12
Constructors of Subclasses 1. the first line in the body of every constructor must be a call to another constructor if it is not then Java will insert a call to the superclass default constructor if the superclass default constructor does not exist or is private then a compilation error occurs 2. a call to another constructor can only occur on the first line in the body of a constructor 3. the superclass constructor must be called during construction of the derived class 13
Mix (version 1) public final class Mix extends Dog { // no declaration of size or energy; part of Dog private ArrayList<String> breeds; public Mix () { // call to a Dog constructor super(); this.breeds = new ArrayList<String>(); } public Mix(int size, int energy) { // call to a Dog constructor super(size, energy); this.breeds = new ArrayList<String>(); } 14
public Mix(int size, int energy, ArrayList<String> breeds) { // call to a Dog constructor super(size, energy); this.breeds = new ArrayList<String>(breeds); } 15
Mix (version 2 using chaining) public final class Mix extends Dog { // no declaration of size or energy; part of Dog private ArrayList<String> breeds; public Mix () { // call to a Mix constructor this(5, 5); } public Mix(int size, int energy) { // call to a Mix constructor this(size, energy, new ArrayList<String>()); } 16
public Mix(int size, int energy, ArrayList<String> breeds) { // call to a Dog constructor super(size, energy); this.breeds = new ArrayList<String>(breeds); } 17
why is the constructor call to the superclass needed? because Mix is-a Dog and the Dog part of Mix needs to be constructed 18
Mix mutt = new Mix(1, 10); Mix object Dog object 1. Mix constructor starts running creates new Dog subobject by invoking the Dog constructor 2. Dog constructor starts running creates new Object subobject by (silently) invoking the Object constructor 3. Object constructor runs sets size and energy creates a new empty ArrayList and assigns it to breeds Object object size 1 energy 10 breeds 1000 19
Mix Memory Diagram 500 Mix object size 1 energy 10 breeds 1000 1000 ArrayList<String> object... 20
Invoking the Superclass Ctor why is the constructor call to the superclass needed? because Mix is-a Dog and the Dog part of Mix needs to be constructed similarly, the Object part of Dog needs to be constructed 21
Invoking the Superclass Ctor a derived class can only call its own constructors or the constructors of its immediate superclass Mix can call Mix constructors or Dog constructors Mix cannot call the Object constructor Object is not the immediate superclass of Mix Mix cannot call PureBreed constructors cannot call constructors across the inheritance hierarchy PureBreed cannot call Komondor constructors cannot call subclass constructors 22
Constructors & Overridable Methods if a class is intended to be extended then its constructor must not call an overridable method Java does not enforce this guideline why? recall that a derived class object has inside of it an object of the superclass the superclass object is always constructed first, then the subclass constructor completes construction of the subclass object the superclass constructor will call the overridden version of the method (the subclass version) even though the subclass object has not yet been constructed 23
Superclass Ctor & Overridable Method public class SuperDuper { public SuperDuper() { // call to an over-ridable method; bad this.overrideme(); } } public void overrideme() { } System.out.println("SuperDuper overrideme"); 24
Subclass Overrides Method public class SubbyDubby extends SuperDuper { private final Date date; public SubbyDubby() { super(); this.date = new Date(); } @Override public void overrideme() { System.out.println("SubbyDubby overrideme : " + this.date); } } public static void main(string[] args) { SubbyDubby sub = new SubbyDubby(); sub.overrideme(); } 25
the programmer's intent was probably to have the program print: SuperDuper overrideme SubbyDubby overrideme : <the date> or, if the call to the overridden method was intentional SubbyDubby overrideme : <the date> SubbyDubby overrideme : <the date> but the program prints: SubbyDubby overrideme : null SubbyDubby overrideme : <the date> final attribute in two different states! 26
What's Going On? 1. new SubbyDubby() calls the SubbyDubby constructor 2. the SubbyDubby constructor calls the SuperDuper constructor 3. the SuperDuper constructor calls the method overrideme which is overridden by SubbyDubby 4. the SubbyDubby version of overrideme prints the SubbyDubby date field which has not yet been assigned to by the SubbyDubby constructor (so date is null) 5. the SubbyDubby constructor assigns date 6. SubbyDubby overrideme is called by the client 27
remember to make sure that your base class constructors only call final methods or private methods if a base class constructor calls an overridden method, the method will run in an unconstructed derived class 28